1. Field of the Invention
The present invention relates to a color printer and color printing method that print a color image on a recording paper by three-color frame sequential printing, in particular relates to a color printer and color printing method that can prevent failure in registering due to a change of conveyance speed of the recording paper.
2. Explanations of the Prior Arts
A color thermal printer is generally known as a kind of color printer. The color thermal printer makes a thermal head heat the recording paper to print a color image. Instead of a cut sheet, a roll paper in which the recording paper is rolled tends to be used in the color thermal printer. The color thermal printer has two types, which are a one-head three-pass printer and a three-head one-pass printer. In the one-head three-pass printer, the recording paper is unwound from a paper roll and rewound thereto alternately. For instance, a single thermal head sequentially records a yellow image, a magenta image, and a cyan image while the recording paper is rewound for three times. Such a color thermal printer has some advantages that the print size is suitably changed in a longitudinal direction of the recording paper as well as reducing the size of the color thermal printer. In the three-head one-pass printer, on the other hand, three thermal heads are arranged at predetermined intervals. Each thermal head records each one of the three primary color images respectively while the recording paper is being supplied from the roll paper.
The front end of the recording paper is pulled from the roll paper by a supply roller that contacts the periphery of the roll paper. After the end of the recording paper is pulled, the recording paper is nipped by feed roller pair, which consists of a capstan roller and a pinch roller, and conveyed in a wind direction and a rewind direction of the recording paper. While the recording paper is being conveyed in the rewind direction, the thermal head heats the recording paper to record one of the three primary color images within a recording area of the recording paper (U.S. Pat. No. 6,154,241 corresponding to JPA No.2000-168114, for instance).
The printing area may be stretched and compressed due to heat of the thermal head. In addition, the fricative force to the feed roller pair may change according to the content of the color image printed within the printing area. This changes the conveyance amount slightly while the printing area of the recording paper is being in contact with the feed roller pair. As a result, uneven density and color registering failure occurs on the printed color image, to cause deterioration of image quality.
In order to cope with failure in registering, measures to prevent heat fluctuation are taken. However, the color registering failure still occurs in spite of this to require actions to take.